Re: [Qemu-devel] macvtap performance: good when writing from guest, abysmal when reading on guest (~ 700kB/s)

[Lutz Vieweg <lvml@5t9.de>]

Adding to my symptom description: I re-tested with "vhost=on" in addition
and verified this feature was actually used.
But this didn't change the benchmark results.

Regards,

Lutz Vieweg

On 01/20/2012 04:10 PM, Lutz Vieweg wrote:
> Hi,
>
> I've been using qemu-kvm along with ordinary tap-devices and software bridges
> for quite some time. When I recently noticed that a certain TCP connection between
> a guest and a remote physical host was limited to ~ 80MB/s, I thought it would
> be a good idea to check whether by using "macvtap", instead, the performance
> would get better.
>
> So I setup a guest on a host that has a direct peer-to-peer 10G cable to
> another host, and configured it to use a macvtap device.
>
> Then I did some benchmarks, using "nc" on both sides, just reading from /dev/zero,
> writing to /dev/null.
>
> When the guest VM is writing into a TCP connection to the physical host (linux-3.1.6),
> the performance is ~ 140MB/s - not great, but better than with ordinary
> tap devices.
>
> But to my big surprise, the performance when the physical host is writing,
> and the guest VM is reading is abysmal, only ~ 700kB/s!
> No bottleneck is obvious - the CPU usage and NIC utilization of both
> the VM, its host, and the other host is all quite low.
> "strace" on qemu process indicates that from time to time, there are "pauses" of ~ 0.5
> seconds in between the many reads from /dev/tapX, but I am not sure whether
> this is the whole reason for the bad performance.
>
> Any ideas?
>
> Or should I rather stay with ordinary tap/brctl, or try yet another
> virtual NIC technique?
>
> Regards,
>
> Lutz Vieweg
>
>
>
>